1. Field of the Invention
The present invention relates to an optical disk apparatus which is capable of writing data to an optical disk and/or reading data from an optical disk. The present invention also relates to an optical element which is suitable for use in such an optical disk apparatus, and a method for producing the same.
2. Description of the Related Art
An optical disk apparatus comprises a motor for rotating an optical disk, an optical pickup which irradiates the optical disk with a light beam, a signal processing section for processing recording or reproduced data, and like elements. Among others, the optical pickup, which is a most vital component to enhanced storage density, comprises a light source for generating a light beam, lenses for converging the light beam onto the recording surface of the optical disk, and a photodetector for detecting light which has been reflected from the optical disk (reproduction light or signal light) and converting the detected light into an electrical signal.
A known optical disk apparatus is disclosed in, for example, Japanese laid-open patent publication No. 2000-132848.
Hereinafter, referring to FIGS. 19A and 19B, the structure of the conventional optical pickup disclosed in Japanese Laid-Open Patent Publication No. 2000-132848 will be described.
FIG. 19A shows an optical pickup structure in a conventional optical disk apparatus. FIG. 19B shows the neighborhood of a light source 1 thereof.
As shown in FIG. 19A, this optical pickup comprises a photodetection substrate 9 on which the light source 1 (e.g., a semiconductor laser) is mounted, as well as an optical system. The optical system includes a collimating lens 4, a polarization hologram substrate 2, a ¼ wavelength plate 3, and an objective lens 5, which are provided along an optical axis 7. The ¼ wavelength plate 3, which is formed on the same substrate as a hologram surface 2a of the polarization hologram substrate 2, moves integrally with the objective lens 6.
The surface of the photodetection substrate 9 includes a region (detection surface 9a) in which a plurality of photosensitive portions such as photodiodes are formed, and a region in which the light source 1 is mounted. As shown in FIG. 19B, a reflection mirror 10 is formed on the surface of the photodetector substrate 9, the reflection mirror 10 reflecting light emitted from the light source 1 in a direction which is substantially perpendicular to the surface of the photodetection substrate 9.
Laser light which has been emitted from the light source 1 is reflected from the reflection mirror 10 on the photodetection substrate 9, and thereafter collimated into parallel light by the collimating lens 4. The parallel light is transmitted through the polarization hologram substrate 2 in the form of P-polarized light. The polarization hologram substrate 2 is characterized so that it does not diffract P-polarized light, but diffracts S-polarized light. In the case where the incident light is S-polarized light, the polarization hologram substrate 2 has a diffraction efficiency of about 0% for the 0th order light, and about 41% for the ±1st order light, for example.
The light transmitted through the polarization hologram substrate 2 is converted by a ¼ wavelength plate 3′ from linearly polarized light (P-polarized light) into circularly polarized light. The circularly polarized light is converged by the objective lens 5 onto a signal surface 6a of the optical disk substrate 6. The ¼ wavelength plate 3′, which is constructed on the same substrate as the hologram surface 2a, moves integrally with the objective lens 6.
The light (signal light) which has been reflected from the signal surface 6a of the optical disk substrate 6 propagates in the opposite direction of the forward path. This light (signal light) travels through the objective lens 5 and enters the ¼ wavelength plate 3′. The light transmitted through the ¼ wavelength plate 3′ is converted from circularly polarized light into linearly polarized light (S-polarized light). The S-polarized light enters the hologram surface 2a of the polarization hologram substrate 2 so as to be diffracted. Through this diffraction, 1st order diffracted light 8 and −1st order diffracted light 8′ are formed with respect to the optical axis 7 as an axis of symmetry. The diffracted light 8 and 8′ is each converged on the detection surface 9a on the detector 9 via the collimating lens 4. The detection surface 9a is located substantially at the focal plane of the collimating lens 4 (i.e., an imaginary emission point on the light source 1).
Generally-used optical disk systems are designed on the premise that the optical disk substrate 6 does not have any birefringence. In reality, however, there are some low-quality optical disk substrates 6 which do suffer from a large birefringence, thus inviting the following problems.
Assuming that the laser light which is emitted from the light source 1 has a wavelength of λ, the birefringence of the optical disk substrate 6 may cause a birefringent phase difference (retardation: phase delay) exceeding λ/2, over the course of the back and forth trips of light. When converted into an angle, λ/2 equals 180°. Hereinafter, any birefringent phase difference will be expressed in terms of angle.
Assuming that the birefringent phase difference ascribable to the optical disk substrate 6 is 180° over the course of the back and forth trips of light, and when taken together with the birefringent phase difference (180°) of the ¼ wavelength plate 3′ over the course of the back and forth trips of light, there is a birefringent phase difference of 360°. As a result, the signal light entering the polarization hologram substrate 2 is P-polarized, instead of being S-polarized. Since the polarization hologram substrate 2 is characterized so as not to diffract P-polarized light, the light in the return path, which is P-polarized, is not diffracted. This means that the light amounts of the diffracted light 8 and 8′ shown in FIG. 19 are zero. Therefore, the photodetector 9 cannot receive the signal light reflected from the signal surface 6a. Thus, not only is it impossible to read the signal, but it is also impossible to perform focusing and tracking controls, etc.